This invention deals with polymerizable cyanoacrylate adhesives, compositions containing as a major active ingredient at least one monomeric ester of 2-cyanoacrylic acid. These compositions are stabilized to prevent premature or spurious polymerization prior to the time of intended use.
Cyanoacrylate adhesive compositions are extremely sensitive, and great care must be exercised in their formulation. Cure (polymerization) is generally considered to be initiated by an anionic mechanism, with water being a sufficiently strong base to initiate cure under most circumstances. The adhesives remain shelf stable items of commerce as long as they are suitably packaged, but when placed on a substrate to be bonded and exposed to atmospheric and surface moisture in so doing, cure generally is instituted in a relatively short period of time, generally in less than 1 minute and on many surfaces in a matter of a few seconds.
Because of their exceptional speed of cure, cyanoacrylate adhesives offer many advantages. To render cyanoacrylate adhesive compositions useful in adhesive bonding, attempts have been made to increase the viscosity of the adhesive. Increased viscosity is desirable and in many cases essential in order to fill larger gaps and to retain the adhesive on the substrates to be bonded prior to cure. Acrylic polymers of various types have been disclosed as useful thickeners, such as poly (ethyl 2-cyanoacrylate) of U.S. Pat. No. 3,564,078 to Wicker et al, issued Feb. 16, 1971, and poly (methylmethacrylate) of U.S. Pat. No. 3,282,773 to Wicker et al, issued Nov. 1, 1966.
Heretofore, however, it has not been possible to prepare acceptable cyanoacrylate compositions of increased viscosity, such as above about 200 centipoise. The major problem has been the stability problems which have been produced above that viscosity level, since shelf stabilities of less than 6 months commonly have been encountered. In addition, when higher levels of thickening agents were incorporated, substantial reductions in adhesive bond strength, in ability to cure in larger gaps between substrates (such as 5 mils or greater), and in optical clarity in the cured adhesive product have been encountered.
The ability to prepare cyanoacrylate adhesive compositions with viscosities of greater than 200 centipoise which do not suffer from the aforementioned shortcomings of the prior art, would be a major advance in adhesive technology and would permit the production of novel and extremely useful adhesive compositions.